1. Technical Field
This invention relates generally to camera calibration, and more particularly to a method of color and/or intensity calibrating a digital image capture device.
2. Description of the Background Art
Digital camera modules are currently being incorporated into a variety of host devices. Such host devices include cellular telephones, personal data assistants (PDAs), computers, etc. And, consumer demand for digital camera modules in host devices continues to grow.
Host device manufacturers prefer digital camera module to be small, so that they can be incorporated into the host device without increasing the overall size of the host device. Further, host device manufacturers desire camera modules that minimally affect host device design. Further, camera module and host device manufacturers want the incorporation of the camera modules into the host devices not to compromise image quality.
A conventional digital camera module generally includes a lens assembly, a housing, a printed circuit board (PCB), and an image capture device (ICD). Upon assembly, the ICD is electrically coupled to the PCB, which is affixed to the bottom of the housing. The lens assembly is adjustably mounted to the opposite end of the housing to focus incident light traveling through the lens onto an image capture surface of the ICD. The PCB includes a plurality of electrical contacts that provide a communication path for the ICD to communicate raw image data received by the ICD to the host device for processing, display and storage.
ICDs are often formed of small silicon chips containing large arrays of photosensitive diodes called photosites. When the shutter is actuated, each photosite records the intensity or brightness of the incident light by accumulating a charge; the more light, the higher the charge. The ICD sends the raw image data indicative of the various charges to the host device, where the raw image data is processed, e.g., converted to formatted image data (e.g., JPEG, TIFF, PNG, etc.) and to displayable image data (e.g., an image bitmap) for display to the user on, for example, an LCD screen.
These photosites use filters to measure light intensities corresponding to various colors and shades. Typically, each individual photosite includes one of three primary color filters, e.g., a red filter, a green filter and a blue filter. Each filter permits only light waves of its designated color to pass and thus contact the photosensitive diode. Thus, the red filter permits only red light to pass, the green filter only permits green light to pass, and blue filter only permits blue light to pass. Accumulating three primary color intensities from three adjacent photosites provides sufficient data to yield an accurately colored pixel. For example, if the red filter and the green filter accumulate a minimal charge and the blue filter accumulates a peak charge, the captured color must be blue. Thus, the image pixel may be displayed as blue.
After assembly, the camera module is typically calibrated to known intensities of light through the color filters. One prior art method includes taking a picture of a color chart (e.g., MacBeth color chart) and running the image data through color correction processes. The recorded intensities are corrected to correspond to the known color intensities. This process can be done relatively quickly, because the color correction can be effected from a single exposure.
The typical color chart is manufactured from colored dyes. Unfortunately, calibration using the typical color chart results in substandard calibration for those colors not present in dyes. The conventionally calibrated camera module has difficulty measuring other natural colors not provided by the color chart.
Some camera module manufacturers calibrate camera modules using a device called a monochromator. A monochromator sends light through a prism to output a predetermined color. Then, a picture of the predetermined color is taken. The camera module is then calibrated to the known intensity of the particular color. The process is repeated for another color, for an estimated 24 colors or more. Although the monochromator facilitates the calibration of natural colors, it has disadvantages. Such devices are relatively expensive. Also, several pictures must be taken, one for each color to be calibrated. This compromises manufacturing throughput, increases time-to-market, and increases overall manufacturing cost.
A system and method for calibrating a digital camera module are needed to increase image quality, reduce time to market, and/or provide effective quality control.